If you've removed a cat there's literally no point- so disabling the super annoying sound is a huge plus.
If you have cams or a built engine, you pretty much have to remove it if you ever want the car to start.
The axle bolts are single-use. There is a "special tool" from BMW to remove the axle from the diff, but I've never actually used it.
Be careful with the suspension bits as always, don't crush anything. It's definitely doable in a drive way. I have a link to the service manual as well. DM me if...
The brace has been tested on several high-power cars over significant of passes with no issues to the diff. The other three mounting points of the diff remain unchanged, but it is recommended you replace the front bushings with solid bushings as well.
The bolts do reuse the factory bolt holes...